The Diocese of Ilagan is a Latin Catholic diocese of the Catholic Church in the Philippines. It was erected on 31 January 1970 from territory of the then Roman Catholic Diocese of Tuguegarao and covered the entire civil Province of Isabela. At the time of Ilagan's creation, Tuguegarao was not a Metropolitan diocese and therefore Ilagan fell under the jurisdiction of Archdiocese of Nueva Segovia. It is a suffragan diocese of the Roman Catholic Archdiocese of Tuguegarao since 1974 when Tuguegarao was elevated to an archdiocese.